Method of making rubbery compounds suitable for production of tire treads



Patented Oct. 20, 1953 METHOD OF MAKING RUBBERY COM- POUNDS SUITABLE FORPRODUCTION OF TIRE TREADS Theodore A. Te Grotenhuis, Olmsted Falls, OhioNo Drawing. Application July 4, 1945,

Serial No. 603,256

1 This invention is a continuation-in-part of my prior applicationsSerial No. 559,848 filed October 21, 1944, now Patent 2,457,097, whichis a continuation-in-part of application Serial No. 424,493 filedDecember 16, 1941, and now abandoned. The invention relates to pneumatictires having wear-resistant treads, compounds for preparing suchwear-resistant treads and to a method of making such compounds. l I

Since the first use of carbon black as a compounding agent for rubber,the quantity added to treadrubbercompounds steadily increased to provideimproved abrasion resistance until it was found that flexing orcrack-resistant properties were adversely afiected. Present day rubbercompounds and tire-treads represent a balance between relativelymediocre abrasion resistance and mediocre flexing or resistance to heatbuildup by flexing. With the advent of synthetic polymerizablematerials, it has often been found that one rubbery polymer may,particularly in the relatively broken-down state, have exceptionallyhigh abrasion resistancewhen it is compounded with suflicient amounts ofcarbon black, yet it may crack badly and have very poor flex-. ingproperties. Another. rubberypolymer may have exceptional anti-crackingand flexing prop-. erties and yet have relatively mediocre abrasionresistance. When blends of rubbery materials are made in accordance withwell-recognized fundamentals of the tire industry, the homogeneousmixture is usually obtained at a substantial sacrifice of propertiesdesired in the polymers. I i

It isan object of the present invention to provide a tire tread whichmay be compoundedflto provide optimum flexing properties and alsooptimum abrasion resistance. g

It is another object of the present invention to provide a method ofmaking tire treads wherein the advantages of two different types ofrubbers or rubber compounds containing carbon black may be retained. I

It, is another object of the present invention to provide a vulcanizablerubberlike material characteristic of vulcanizable rubber; andcontaining dispersed vulcanizable rubber or synthetic rubbery compoundwhich when vulcanized has abrasion resistance superior to that of thematrix compound, which is also vulcanizable to In the preparation ofmolded rubber articles .such as tire treads and the like, it has hereto-4 Claims. (01. 260-5) 50 a resilient material characteristic of softrubber.

will process or tube in a desirable manner to form a 'smooth'surface.Such break -down or mastication, as is well-known, causes great loss ofsome of the most desirable properties of the rubbers or rubberypolymers. Abrasion resistance, for example, may be reduced bymastication as much as or 50% of that inherent in the unmasticated orslightly masticated materials.

It is still another object of the present invention to provide arubberlike compound wherein all of the rubbery material therein need notbe broken down to increase processability so that it can be formed inthe desired shape.

It is a primary object of the present invention toprovide a compoundsuitable for tire treads which may be formed in the desired shape, whichis vulcanizable to provide resilience similar to that of vulcanizedrubber and which retains the advantageous properties of rubber or arubbery polymer which has not been appreciably deteriorated bymastication.

These and other objects are accomplished by mixing two separatelyprepared rubber compounds, one compounded to provide high abrasionresistance and the other compounded to provide superior flexingproperties in such a way as to produce a heterogeneous compound havingparticles of the relatively high abrasion-resistant polymer as adisperse phase and having the relatively elastic flexible rubberypolymer as a matrix or connecting ingredient.

The rubbery compounds of the present invention are preferably formed byseparately preparing the compounds intended respectively for disperseand continuous phases. The compound intended as the matrix should besubstantially less plastic then the compound intended as the dispersephase. The plasticity may be measured with the recognized Mooneyshearing disc plastometer or with an extrusion plastometer, such as aDillon-Johnston plastometer, manufactured by The Firestone Tire & RubberCompany. A difference in plasticity between the two rubber compounds ofat least ten points, and preferably substantially twenty or thirtypoints or more on 2.. Mooney plastometer, is desirable. The

Mooney plastometer is described in an article entitled A shearing discplastometer for unvulcanized rubber by Melvin Mooney, published inIndustrial and Engineering Chemistry, Anal. Ed, 6, 147 (1934). Themeasurement of plasticity or plasticity standards are also described inthe Government Specification for Synthetic Rub- Ju includingmethacrylylurea and vinylketones,

substituted vinyl ketones, unsaturated, low molecular weight acids, suchas fumaric acid, crotonic acid, their esters and amides, with or withoutpolyhydric alcohols, mixtures of vinyl amide and phthalic anhydride orvinyl phthalamide, etc. may also be present in the monomeric mixture tomodify the properties of the polymer.

It is, of course-understood that the separate ingredients of thesecomplex rubbers, such as the dienes or the copolymerizable unsaturatedcompounds, mayby themselves be partially but incompletely polymerizedbefore H being mixed with the other monomers without departing from theinvention. Such partially polymerized dienes and olefins are hereindesignated and included by designating their respective monomeric sub-.

stances.

It is to be noted that both the disperse phase and the matrixcomposition of the vulcanized articles of the presentinvention haveproperties of the general characteristic of soft, vulcanized rubber. Thevariations in the respective proportions and number of the monomericmaterials necessary for the attainment of such properties in thecopolymer are almost infinite. It is well-known or apparent to thoseskilled in the art that one copolymerizable material may be substitutedin whole or in part by another or by two or more others to obtainimprovements in two or more specific properties. The total amount ofconjugated dienes should usually constitute a major proportion (at least50%) of the mixture of polymerizable unsaturated materials used informing polymers. However, in some instances, as in the caseofomethylacrylate, etc. when the copolymerizable, unsaturated materialalone produces'on polymerization a relatively soft polymer, vulcanizablerubbery materials are obtained with a somewhat smaller amount (only 20%or 25%) of conjugated dienes, and in the case of Butyl rubber the amountof conjugated diene may be as low as 1% or even less to give the desiredvulcanizability. It is generally preferred. that the copolymerizableunsaturated materials be present in amounts between 10 and 40 or 45% ofthe monomeric mixture, the characteristics of copolymers of thesematerials and the effect of variations in quantities beingwellknown tothose skilled in the art.

Although rubbery copolymers, prepared from monomers containingconjugated dienes are preportions of the polymerizable materials in,the: monomers used for preparing the rubbery. poly-.

mers or to substitute the polymerizable materials by other polymerizablematerials as they become available. The proportions and the number ofingredients polymerizable to a rubbery mass will be hereafter producedand are polymerizable to rubber powme s. Theo ymers sho l howr ever,preferably be of the type which are" vulcanizable either with sulfur orother agents;

capable of converting the material, withgor with-: out heat, from theplastic to the elastic state.: Even rubbery products produced bycondensa-' tion (rubbery condensation polymers), such for example ascertain rubbery reaction products of phase of the compounds of thepresent inven-l tion. It is thusseen that the term "vulcanizablevsynthetic rubberlike material as used herein .in its broadest scope isentitled to include sulfurvulcanizable synthetic rubbers, as well asother. synthetic vulcanizable materials of a rubbery resiliencecharacteristically resembling after vulcanization a soft vulcanizednatural rubber.

Although halogen-substituted dienes, such for.. example as chloroprene(chloro-2-butadiene-1,-3)

do not polymerize into a rubber generally considered capable of beingreinforced by carbon black, the latter material does stiffen and improvethe characteristics of 'polychloropr'en'e compounds. chloroprene, areadvantageous, however, in that they may be polymerized directly to thevul-x canized or cured state without the addition of curing agents.

The matrix or continuous phase portion of the mix may be any compoundvulcanizable to a soft, fiexible rubbery state similar to softvulcanized It is preferably compoundedwith sub-- rubber. stantialamounts of carbon black and compounds generally similar to thoseheretofore used for preparing tire treads and containing the usualcompounding ingredients, such as curing agents, pigments, activators,plasticizers, stearic acid,

antioxidants, accelerators, etc. are usually used. The vulcanizingagents may be present in suf-' ficient quantity to effect the desiredcuring of the disperse phase or, if desired, the disperse phase may evenbe at least partially vulcanized to an elastic state prior to formingthe hetero' geneous mixture.

Superior results may be obtained when the rubbery binding material ofthe matrix is plasticized, wholly or partly, with a polymerizable liquidor semi-solid which is capable of being further polymerized preferablyto a rubbery- Such a polymerizable material by further polymerization inthe matrix tends to greatly strengthen the bond between the dispersedglob polymer.

ules and the matrix compound, so that the globules are firmly in thematrix and are not 5 readily displaced or removed bodily by abrasion.

The polymerizable liquid plasticizers preferably used may be prepared bypartially -poly-' merizing one or more suitable monomeric materials,partial polymers, or mixtures thereof, etc.

"to. obtain a relatively non-volatile polymer whichmay be a relativelysyrupy, gellike or viscous:- liquid, or a highly plastic solid,oramixture of su-ch liquids and/or solids, It may be mixed with carbonblack prior to its admixture with in gredients forming the matrix. Thus,for example, it may be prepared by partly condensing an alkylphenol, forexample,tertiary butylphenol with an aldehyde such as formaldehyde orbutyral aldehyde,. etc., in alkalinesolution to produce'a;

These haloprenes, particularly.

carbon black is incorporated in latex, however, in amounts in excess of20 or 30 parts per 1 parts of rubbery material, it is usually desirablethat the coagulum be subject to a small amount of shearing or deformingtreatment prior to complete drying for the reason that the carbon blackupon coagulation in the latex-carbon black mixture tends to formagglomerates of unduly high concentrations of carbon black. The shearingtreatment of the agglomerate may be after partial drying, if desired,but water should ordinarily be present in amounts of at least .5% or 1%,based on the rubber of the mixture, and the shearing of the undriedcoagulum between sur faces at different speeds may be before drying, asmay be accomplished in a pelletizer, utilizing extrusion through anorifice or on a rubber mill if preferred.

A proportion of the higher loaded less plastic polymer within theheterogeneous compounds of the present invention may be varied to obtainthe desired properties for the intended use. The proportion should notbe so large that the particles cannot be bound together by the moreplastic matrix compound. The greatest abrasion resistance is usuallyobtained when the amount of the matrix is just suflicient to fill thespaces between the separate particles of comminuted materials which arepacked together or substantially in contact. Generally, the amount ofdisperse phase is equal to or greater than the amount of the matrix, butcompounds in which the disperse phase varies from 20 or 30% to 80 or85%, by weight, of the heterogeneous compounds are desirable forcommercial use.

The following examples, in which parts are by weight, illustrate thepresent invention.

Example 1 To form a rubber compound for the disperse portion of aheterogeneous compound, 80 parts of medium processing channel black, 5parts of zinc oxide, and about /2 part of sulfur are dispersed in about400 parts of water and about parts of Daxad (condensation product offormaldehyde and naphthalene sulfonic acid) by passing the aqueouscarbon black mixture through a colloid mill. The carbon black mixturethus formed is incorporated into about 400 parts of GR-S latexcontaining about 25% of rubber solids. The mixture thus produced iscoagulated in conventional manner with salt and acetic acid to produce afine crumb, which is filtered from most of the excess water, passedthrough fine orifices of a pelletizing tuber to cause smearing of theblack and formation of small pellets. The pellets are then dried toabout 5% moisture content. Samples when compacted have a Mooneyviscosity somewhat above 80, a Williams viscosity around 440, and areadin of about 4.5 on the extrusion (Dillon) plastometer.

To form a matrix compound, a homogeneous mix of the followingingredients is prepared:

=(Butadiene-styrene 70:30 ratio, only partially .10 polymerized andhaving a viscous syrupy-tohighly plastic solid, Mooney plastometerreading of 10 or less.)

The rubber is sufiiciently broken down to band on a mill in the usualmanner and plasticized by the pine tar and the syrupy polymer, added alittle at a time in the usual manner, or if desired the syrupy materialis first mixed with the carbon black and incorporated into the rubbersimultaneously therewith. The other ingredients are added in theordinary manner. The Mooney viscosity of the mix is less than 30,corresponding to an extrusion rate of about 1 second on the extrusion(Dillon) plastometer.

With the mill rolls separated just sufficiently to form a small bank ofrubber, the pellets previously prepared are added a little at a time tothe bank of rubber and by the mastication caused to be enveloped in themass of the matrix compound to form what may be designated as a roughmix. As the pellets disappear, more are added, until about equal partsby weight of the pellets and the matrix compound are present. Half ofthe compound thus formed is removed from the rolls and the mill rollsthen brought considerably closer together to insure break-up of theparticles into a finely divided state. After, the batch on the mill isremoved and shaped by extruding, calendering or the like to a formdesirable for tire treads. It then may be applied to the tires toprovide high abrasion resistance coupled with relatively low runningtemperatures. After removal of the first batch from the mill, a secondbatch may be similarly milled to insure the presence of only relativelyfine particles of the disperse phase. The mass is distinguishable from ahomogeneous mix by microscopic examination and to some observers byvisual observation of a film stretched over a light source.

Example 2 The crumb oi the preceding example is dried to about 5%moisture content and sheeted through a mill to smear the black. Ribbonscut from the sheets are fed slowly or in small increments (compared tothe mass of the matrix compound) to the bank of the afore-mentionedhighly plastic matrix compound on the mill in place of the pellets ofExample 1. Substantially equal results are obtained.

Example 3 A dispersion of about 35 parts of easy processing channelblack, 30 parts medium processing channel black, 5 parts of high modulusfurnace black, 1 part of an accelerator (CaptaX), and 5 parts of zincoxide in about 100 parts of GR-S is made by preparing a dispersion orslurry of carbon black in about 350 parts of water and about 5 or 10parts of quebracho, mixing the carbon black dispersion thus preparedinto about 400 parts of GR-S latex containing about 25% of rubbersolids, all as in Example 1, coagulating the mixture, sheeting out thecoagulum between rolls, and drying to about 1% moisture content or so.Samples of the dried coagulum after slight mastication have a Mooneyviscosity in the neighborhood of and an extrusion time of about 4.2seconds on the Firestone (Dillon) extrusion plastometer after only aslight mastication. The compound is sheeted and cut in the form of anarrow ribbon for addition to a portion of the more plastic matrixcompound prepared of the following ingredients, substantially asdescribed in Example 1:

The heterogeneous compound is prepared by slowly adding to two-thirds ofthe above matrix compound all of the above highly loaded channel blackcompound, which has not been. masticated an appreciable amount, to causedeterioration of the rubbery base. The ribbon of the highly loadedcompound is distributed slowly over the bank of rubber matrix compound,taking care that substantially all of the first addition at any point onthe mill is enveloped or surrounded by the matrix before subsequentaddition is added. After formation of this rough. compound, the millrolls are tightened to insure breaking up of the high carbon compoundinto small agglomerate's of the order of or less average diameter. Thecompound thus prepared is readilytubable, even though the high carbonblack loaded compound is relatively unbroken-down and has thecorresponding high abrasion resistance when cured. Tire treads formedfrom the compound have relatively low heat build-up, good flexing andcracking resistance, combined with relatively high abrasion resistancecompared to homogeneous tire treads.

Example 4 When the rough mix. of Example 3 is prepared by adding theribbon of. highly loaded carbon black compound in successive incrementsto the Banbury containing the more plastic heterogeneous compound,andthe rough mixture from the Banbury subsequently milled on arelatively tight mill to breakup the agglomerates, a compound of verydesirable properties is obtained.

Example 5 When in Example 1 the GRI-S latex is substituted by an equalamount of a latex of a rubbery polymerization product, obtained bypolymerizing a liquid mixture of about parts of dichlorostyrene, 10parts of styrene, and '70 parts of butadiene in emulsion form so as toobtain about rubbery solids, the other steps and ingredients remainingthe same, a desirable tire tread is also produced.

Example 6 When the natural rubber in the matrix compound of Example 1 issubstituted by an equal weight of a sulfur-vulcanizable syntheticrubber, such as GR-S, the sulfur content simultaneously being reduced toabout 2 parts, other conditions and the procedure of Example 1 remainingthe same, the tire tread produced by extruding the heterogeneous mix andvulcanizing on a tire carcass in a suitable mold has relatively highabrasion resistance but somewhat higher heat buildup than the product ofExamples 1 or 3 in which the matrix is a natural rubber.

Example 7 The GR-S latex in Example 3 is substituted by natural rubberlatex, the HMP carbon black is omitted from the matrix compound, theother conditions remaining the same. A tire'tread hav- 12 relativelygood abrasion resistance is thus obtained.

Example- 8' The highly'loaded carbon. black composition. for thedisperse phase of the heterogeneous compound of Example 3 is substitutedby a compound prepared by milling the same proportions of carbon blackand other ingredients into solid natural or synthetic rubber sheetedinto ribbon form and mixed into the matrix composition. Theheterogeneous compound prepared has properties desirable for thepreparation of tire treads.

Example 9 The uncoagulated, well-dispersed latex-carbon black mixture,identical with that of Example 3, is spray dried in accordance withusual procedure to produce a relatively fine powder, which is added alittle at a time to the matrix composition of Example: 3. Theheterogeneous compound produced has very low heat build-up and hasexcellent properties.

The matrix composition may be substituted by other rubber compounds orcompounds of vulcanizable natural or synthetic rubbers, as above setforth and as set forth in the afore-mentio'ned prior applications, ofwhich this application is a continuation-in-part.

In the above examples, a rubber or rubbery material of the dispersephase may be substituted by any one rubbery material or by a blend ormixture of one or more rubbery polymers, or even by natural rubber, asabove set forth. It is preferably compounded with most or all of thedesired compounding and curing agents, as well as carbon black, althoughit is found that the curing agents may usually be omitted as sufficientmigration from the matrix compound in the disperse phase is usually had,particularly if the particles of the disperse phase are less than oraverage diameter, as present in compounds having the more desirableoperating characteristics.

An important feature of the heterogeneous compounds of the presentinvention is the blending of the interface of the disperse and matrixphases, which as adore-mentioned is accomplished both by the softeningeffect of. the matrix phase and the mixing and shearing operations inthe composition. This gradual rather than abrupt change in properties inthe region of the interface of the two compounds obviously cannot occurto any desirable extent if the disperse phase is vulcanized prior to theaddition. Hence, vulcanization of the disperse phase prior to additionyields a product having properties which are much inferior to those ofcompounds prepared by milling an unvuleanized rubbery compound or powderinto a vulcanizable matrix, although such a product has substantialutility.

It will be seen from the above that the disperse phase need not bedeteriorated by mastication to permit relatively easy processing of thetread compound as the particles of the disperse phase in shapingoperations readily move bodily through the matrix composition which isof relatively lower plasticity. In the case of a homogeneous compound,on the other hand, the mastication 0r plasticization of the entirecompound results in great loss in the wear resistance of the tread, sothat many of the desirable features naturally inherent in the rubber aregreatly deteriorated by the necessity for the attainment of processingproperties in manufacing even lower heat build-up but still retaining.turing operation The separate globules of high carbon black tion whenthe aforementioned 'interfacial gradient is present also apparently actas stops to inhibit crack growth, as well as act as the highWear-resistant elements of the composition, so that the tire treads orother articles which may be formed therefrom have high abrasionresistance due'to the disperse phase and relatively low heat build-updue to the matrix composition. The term "polymer of a diolefiniccompound? in the appended claims is used in the generic sense to includecopolymers of a diolefinic compound with other polymerizable materialsas well as solids obtained by polymerizing a liquid consisting of adiolefinic compound alone.

It is to be understood that variations and modifications of the specificproducts and processes herein shown and described for purposes ofillustration may be made without departing from the spirit of theinvention.

What I claim is:

1. A method of preparing a plastic heterogeneous compound for theproduction of tire treads and the like which comprises incorporatingcarbon black into a latex of a vulcanizable waterinsoluble polymer,coagulating the mixture thus obtained to form a plastic compound havinga base of said polymer and containing substantial amounts of carbonblack, separately forming a continuous phase of more plastic matrixcompound having a base of a vulcanizable waterinsoluble polymericmaterial and also capable of being vulcanized to a soft rubbery state,dispersing particles of the separately mixed less plastic compound intoa continuous phase of said more plastic matrix compound, and masticatingthe resultant mix to cause blending of the interface between particlesof the less plastic compound and said matrix compound, both of saidcompounds when vulcanized being characteristic of a soft vulcanized, asdistinguished from a hard vulcanized, rubber compound, the amount ofcarbon blacks present in the less plastic of said compounds beinggreater than the amount in said more plastic compound, said polymer andsaid polymeric material being selected from members of the groupconsisting of plastic natural rubber and polymers of a conjugateddiolefinic compound having less than 3 carbon atoms, the differencebetween plasticities of said compounds being at least 20 points on aMooney plastometer.

2. A method of making a plastic, vulcanizable compound suitable forpreparing articles havin portions with a flexibility generallycharacteristic of soft, vulcanized rubber compounds which comprisespreparing a matrix compound of a vulcanizable, water-insoluble polymericmaterial in a plastic form, and incorporating in a continuous phase ofsaid vulcanizable matrix compound, a little at a time compared to theamount of said matrix compound and at insufficient rate to form saidless plastic polymer into a continuous phase, a substantial propor: tionof a less plastic, compounded, vulcanizable, water-insoluble polymer,which compounded polymer while plastic has a Mooney plasticity readingof at least twenty points more than the Mooney plasticity reading ofsaid matrix, said incorporation being accomplished by adding said lessplastic polymer to a continuous phase of said more plastic matrixcompound while the latter is being masticated, and masticating thematerials together to form a mixture, whereby discrete particles of saidpolymer are distributed as discrete particles in said matrix and aninterface of intermediate composition is formed by mingling of saidpolymer particlesand said matrix, said polymer and said polymericmaterial being selected from the group consisting'oi plastic naturalrubber and plastic vulcanizable polymerization products 'of a materialcomprising a polymerizable diolefinic compound of less than 8 carbonatoms, said polymer and said polymeric material having differentcompositions L 3. A method of 'making a plastic vulcanizable compoundsuitable for forming rubberlike articles having high abrasionresistance, which comprises masticating a rubber compound to increaseits plasticity, and then gradually incorporating in a continuous phaseof said compound a carbon black stiffened, homogeneous masticatedplastic rubbery polymerization product of a mixture comprising butadieneand styrene, said carbon black stiffened rubber polymerization producthaving when mixed with said rubber compound, a Mooney plasticity readingof at least 20 points more than the Mooney plasticity reading of saidrubber compound said incorporation being accomplished by adding saidcarbon black stiffened plastic polymerization product to a continuousphase of said matrix compound during the mastication of the latter andat a rate insufiicient to form said added compound into a continuousphase, whereby the less plastic polymerization product is broken up anddispersed as particles through said matrix and an interface of graduatedand intermediate physical properties is formed between said matrix andsaid particles by blending of material from the surface regions only ofsaid particles with material of said matrix by mastication.

4. A method of making a plastic rubbery compound suitable for theproduction of tire treads and the like, having when vulcanizedflexibility generally characteristic of tire tread compounds, whichcomprises preparing a relatively plastic masticated matrix of avulcanizable water-insoluble rubber compound, preparing a separatehomogenous masticated mixture of carbon black and a rubbery polymer of aconjugated diolefinic compound, which mixture is substantially lessplastic than is said matrix compound and has a Mooney plasticity readingof at least 1-0 points more than said matrix compound, forming a roughcompound containing said pieces of less plastic homogenous mixtureheterogeneously distributed through said matrix compound by addingpieces or pellets of said less plastic compound to a continuous phase ofsaid matrix, the rate of addition of said pieces or pellets being suchthat they are masticated into said matrix but being insufficient to formthem into a, continuous phase displacing said matrix, and thereaftermilling said rough compound to break up said pieces or particles intosmaller pieces or particles and to form an interfacial region ofgraduated and intermediate hardness therebetween.

THEODORE A. TE GROTENHUIS.

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OTHER REFERENCES 10 Davey, Joun pc. Chem. 1115., p. 515T to 517T,

November 13, 1925.

1. A METHOD OF PREPARING A PLASTIC FHETEROGENEOUS COMPOUNDS FOR THEPRODUCTION OF TIRE TREADS AND THE LIKE WHICH COMPRISES INCORPORATINGCARBON BLACK INTO A LATEX OF A VULCANIZABLE WATERINSOLUBLE POLYMER,COAGULATING THE MIXTURE THUS OBTAINED TO FORM A PLASTIC COMPOUND HAVINGA BASE OF SAID POLYMER AND CONTAINING SUBSTANTIAL AMOUNTS OF CARBONBLACK, SEPARATELY FORMING A CONTINUOUS PHASE OF MORE PLASTIC MATRIXCOMPOUND HAVING A BASE OF A VULCANIZABLE WATERINSOLUBLE POLYMERICMATERIAL AND ALSO CAPABLE OF BEING VULCANIZED TO A SOFT RUBBERY STATE,DISPERSING PARTICLES OF THE SEPARATELY MIXED LESS PLASTIC COMPOUND INTOA CONTINUOUS PHASE OF SAID MORE PHASE MATRIX COMPOUND, AND MASTICATINGTHE RESULTANT MIX TO CAUSE BLENDING OF THE INTERFACE BETWEEN PARTICLESOF THE LESS PLASTIC COMPOUND AND SAID MATRIX COMPOUND, BOTH OF SAIDCOMPOUNDS WHEN VULCANIZED BEING CHARACTERISTIC OF A SOFT VULCANIZED, ASDISTINGUISHED FROM A HARD VULCANIZED, RUBBER COMPOUND, THE AMOUNT OFCARBON BLACKS PRESENT IN THE LESS PLASTIC OF SAID COMPOUNDS BEINGGREATER THAN THE AMOUNT IN SAID MORE PLASTIC COMPOUND, SAID POLYMER ANDSAID POLYMERIC MATERIAL BEING SELECTED FROM MEMBERS OF THE GROUPCONSISTING OF PLASTIC NATURAL RUBBER AND POLYMERS OF A CONJUGATEDDIOLEFINIC COMPOUND HAVING LESS THAN 8 CARBON ATOMS, THE DIFFERENCEBETWEEN PLASTICITIES OF SAID COMPOUNDS BEING AT LEAST 20 POINTS ON AMOONEY PLASTOMETER.